Advanced Transition Metals Flashcards
What orbitals in an octahedral complex have the highest energy
Orbitals along the axis
Dx^2-y^2
Dz^2
Order of orbitals in a square planar complex
Dx^2-y^2
Dxy
Dz^2
Dxz dyz
Tetrahedral energy level order
Dxy dxz dyz
Dz^2 dx^2-y^2
Describe the MOs of an octahedral complex with six sigma donor ligands
6 ligand orbitals and 9 metal orbitals
6 bonding 6 anti bonding and 3 non bonding
The optimum number of valence electrons
Between 12 and 18 valence electrons
Rules of electron counting:
1) metal valence electrons
2) for ligands
3) oxidation state
1) group number is the number of valence electrons of the metal
2) L-type are 2 electron donors
X-type are 1 electron donors
Z-type are 0 electron donors (Lewis acids)
3) oxidation state of metal is the number of x type ligands
Describe transmetallation
Synthesis method for metal alkyl/aryls
Bigger difference in electronegativity increases rate of reaction but decreases the amount of selectivity
Stereochemistry, temperature and concentration also affect reaction
Reactants include RLi RMgX ZnR2 AlR3
Describe electrophilic attack as a synthesis route for metal alkys
R+ reagent
Nucleophilic metal compound required
Eg Na[Mn(CO)5] + MeI goes to
[Mn(Me)(CO)5] + NaI
Describe oxidative addition as a synthesis route for metal alkyls
Often observed in d8 sq planar complexes as they are most vulnerable along the perpendicular to the plane of the molecule
Literally just add things in e.g. MeI
They will be cis to each other
Normally 16 e- but can be 18 if prior de coordination occurs
Describe 2 methods of synthesis for fisher carbenes
1) nucleophilic attack at carbonyl ligand
Very versatile
A range of heteroatoms can be introduced
2) electrophilic abstraction from a metal alkyl complex
Using Me3SiCl
2 methods of synthesis for schronk metal complexes
1) alpha hydrogen elimination from a dialkyl precursor
The bulky ligand means the process is driven by steric hindrance
2) alpha deprotonation of a metal methyl complex
Use NaOMe
If a metal carbonyl has a lot of backbonding, where is it susceptible to attack
Electrophilic attack at oxygen
If a metal carbonyl has poor backbonding where is it susceptible to attack?
Nucleophilic attack at carbon
What type of ligands are alkenes
L-type
2 effects of backbonding to an alkene
Increases carbon carbon bond length
Reduces angles around carbon centres as sp3 contribution increases
Is a metal-alkenyl or metallacyclo structure more reactive?
Alkene is d+ and therefore is susceptible to nucleophilic attack
Increased back bonding in the cyclic compound reduces the charge so the metallocyloalkane is less reactive
A metal carbon sp3 bond describes what type of compound
Metal alkyl including cycles
Metal carbon sp2 bonds are found in what 3 types:
Metal-aryl
Metal-alkenyl
Metal-acyl
Metal carbon(sp) bonds are found in which 3?
Metal carbonyl
Metal alkynyl
Metal isonytryls
Oxidative addition as a main reaction:
Oxidation number, coordination number and VE count all increase by 2
Metals with low oxidation states
Can have concerted, stepwise, radical or ionic mechanisms
Reductive elimination
Oxidation state, coordination number and VE count all decrease (-2)
Metals in medium or high oxidation states
Non polar 3 centre transition state
Sigma metathesis
Typical of early d-block elements in high oxidation states
No change in oxidation state
4 membered cyclic transition state
Concerted process
Ligand substitution
D electron count, coordination and d electron count remain unchanged
Can be associative or dissociative
Migratory insertion
1,2 and 1,1 insertion
No change in oxidation state
Ligands must be cos
1,2 insertion: an unsaturated L type ligand is inserted into a sigma M-X bond after the unsaturated ligand has associated to the metal
1,1 insertion: typical of a carbonyl complex, an x ligand migrates to a coordinated co to form an acyl, leaving one vacant site
Hydride elimination reactions
Reverse of insertion
No change in oxidation state
Alkene often eliminated
Nucleophilic attack of coordinated ligand
Unsaturated organic compounds are activated towards nucleophiles when coordinated to electron deficient metals
Causes elimination of alkene
Electrophilic abstraction
Alkyl or hydride ligands might be abstracted by strong electrophiles such as B(C6F5)3
Coordinated alkane spectra:
Proton NMR
Do not differ drastically
Dependant in metal, oxidation state and other ligands
Coordinated alkane spectra
C NMR:
Both positive and negative chemical shifts
For early metals the signals are more downfield due to deshielding (the opposite is true for late transition metals)
Coordinated arene C NMR
For metal aryls the sigma bound ipso carbon tends to be more deshielded than the corresponding arene
Arene substituents also affect chemical shift, depending on donating and withdrawing effects
Increasing kinetic stability of M-C bonds
Thermodynamically stable but there are many reaction pathways so kinetically unstable
Coordinatively saturated complexes with no Kabila ligands are more stable
Increasing steric hindrance with bulky ligands also increases stability UNLESS PREVENTING ELIMINATION
Beta hydride elimination reactions and 3 things you need:
Mainly metal alkyls
Driving force is the formation of a stronger M-H bond and generation of an alkene (reduces the unsaturation of the metal)
Need:
B-hydrogens
Vacant sites
M-c-c-h syn-coplanar arrangement possible
How can beta hydride elimination reactions be avoided:
Using ligands with no b-hydrogens
Using metals with no empty coordination sites
Small metallacycles show higher stability as there is no syn-coplanar orientation. The smaller the cycle the more stable it is but they still react when heated
Double bonds to bridgehead carbons are unfavourable
Alpha hydride elimination reactions
Observed when B-H elimination is not possible
Mo and Ta are most likely to react in this way
Alpha hydrogens must be available
Free coordination site cis to CH3
Product is very reactive and will proceed to react with any nucleophile
All elimination favoured more as steric hindrance around metal increases
Benzyne formation
Not common due to instability of products
CH bond activation process
High reaction temps and increasing steric hindrance around a metal
Can go on to form many organic compounds with stereochemical control due to insertion step
Alkyne lumo is very low therefore acts as an electrophile
Reductive elimination
Reverse of oxidative addition
Two groups in a cis arrangement
More common for alkyl and square planar complexes
Prevented by strongly chelating ligands and restrictive geometries
Common for biaryl decomposition, where the reactant molecule can be free or bound to the metal centre
Cross coupling catalysts:
1) classics [Pd(PPh3)4] , [Pd2(dba)3] or Pd(OAc)2 + PPh3
Palladium (II) can be used as a pre catalyst and be reduced in situ
2) More sophisticated ligands leave very high turn over frequency and turn over number under mild conditions
How to promote cross coupling:
Strong sigma donating ligands are essential as they facilitate the oxidative addition step and help avoid catalyst deactivation by stabilising low coordinated palladium centres
Steric hindrance also has beneficial effects:
Promotes reductive elimination
Offers kinetic stability of low coordinated centres
Hydrogen activation via oxidative addition
Either dihydride homolytic activation or monohydride heteolytic activation
Reduced pKa of H2 to between 0 and 20 from 35
Agnostic interactions:
3c-2e bonding in electron deficient metals
Can be alpha or beta if hydrogens are available
Can lead to cyclometallation
Characterisation of agostic interactions
1) H atoms not detected by X-ray crystallography so neutron diffraction must be used
2) proton NMR is often unclear and requires the solid state
3) IR has lower C-H stretching frequencies but they are hard to distinguish from other signals
Characterisation of Metallicycles
1) H NMR
Signals at low frequencies (-5 to -45)
Non equivalent hydrides will couple
Coupling to phosphines can determine structure
2) IR M-H stretching frequencies in the range 1500-2200 cm^-1 but usually weak
3) clear X-ray distortions observed
Why is functionalisation of C-H bonds a problem?
1) Alkanes are inert due to strong localised bonding
2) when alkanes react it is at high temperatures or with very reactive compounds, which is unselective
3) formed products are always more reactive than the alkane which can lead to overreaction
C-H oxidative addition
Thermodynamically unfavourable due to strength of M-H vs M-C bonds
Products are prone to reductive elimination
Most activation is aromatic or involves agnostic interactions
Cyclometallation and aryl ligands
Activation of ortho substituents is very easy
The outcomes are either
1) formation of a metal hydride with a higher oxidation state
2) cleavage of an anionic ligand
Intermolecular Oxidative addition
1) C-H activation of arenes:
Good synthesis strategy of metal aryl complexes
2) can also allow the C-H activation of alkanes
Sigma bond metathesis and C-H activation
Typical of early metals with a d0 configuration (as no OA can occur)
Concerted reaction via 4 centre TS
LM-R + M’-R’ to LM-R’ + M’-R
From d2-d10 both OA and SBM are permitted and often the mechanism can’t be confirmed
Can also be used for H2 and in rare cases for C-C bonds
Metaloradical activation
Dimetallic complexes (typically Rh) exist in equilibrium with the monomeric complexes
These react with R-H to form two new complexes
Methane is the most active compound for this
Kinetic control- when toluene is used there is no aromatic C-H activation
Electrophilic Activation:
The two different mechanisms
Can either proceed by:
1) ligand as internal base-
M coordinates to R-H bond then the bong breaks, with metal coordinating to R and XH. XH then dissociates
2) preactivation with external base
M has 2+ charge and coordinates to the RH bond, then an external base reacts with H leading to MR and HX IN ONE STEP
Electrophilic activation:
Displacement of H by a metal:
[M+] + RH > [M]R + H
Typical of cationic complexes of strongly electrophilic metals in normal to high ox states (Pd(ll) Pt(ll) Pt(IV) Hg(ll) and Ti (III)
Often carried out in strongly polar media such as water or strong acids
